The present invention concerns an exhaust gas purifying system provided with a NOx occlusion reduction type catalyst for removing NOx in the exhaust gas of an internal combustion engine such as a diesel engine and a sulfur purge control method in the exhaust gas purifying system.
Various researches and proposals have been made concerning the catalyst type exhaust gas purifying system for reducing and removing NOx from the exhaust gas of an internal combustion engine of an automobile, a floor type internal combustion engine and the like. In particular, NOx reduction type catalysts and three-way-catalysts are used for purifying the exhaust gas of an automobile and the like.
NOx occlusion reduction type catalyst is one such catalyst. This catalyst fulfils its ability of NOx occlusion or its ability of NOx release and removal based on the oxygen concentration in the exhaust gas. For this catalyst, a porous catalyst coat layer of alumina (Al2O3) or the like supports a catalyst metal that oxidizes NOx and a NOx occluding material that occludes NOx. As for this catalyst metal, platinum (Pt), palladium (Pd), or the like can be utilized. On the other hand, the NOx occluding material is composed of any one or several in combination of alkali metals, alkaline-earth metals, rare-earths and the like. These alkali metals include sodium (Na), potassium (K), cesium (Cs) and the like. The alkaline-earth metals include calcium (Ca), barium (Ba) and the like, while the rare-earths include yttrium (Y), lanthanum, and the like.
Now, NOx removal by the above described NOx occlusion reduction type catalyst will be explained.
In an exhaust gas condition in which the oxygen concentration in the exhaust gas is high (lean air/fuel ratio state) as in the normal driving state of diesel engine, lean-burn gasoline engine and the like, the exhaust gas is cleaned as shown in FIG. 4. Nitrogen monoxide (NO) to be discharged is oxidized with oxygen (O2) which is present in the exhaust gas by the oxidizing ability of a catalyst metal 21 and 22, and becomes nitrogen dioxide (NO2). Next, this nitrogen dioxide (NO2) is occluded in a NOx occluding material 23 in the form of nitrate. As a result, the exhaust gas is cleaned.
However, when this occlusion of NOx continues, the NOx occluding material 23 such as barium transforms into nitride and is gradually saturated. Consequently, the NOx occluding material 23 loses its ability to occlude nitrogen dioxide (NO2). Therefore, driving conditions of an engine are changed and the rich-burn is performed generating exhaust gas (rich spike gas) of low oxygen concentration, high carbon monoxide concentration, and high exhaust temperature and delivering the gas to the catalyst.
In this rich air-fuel ratio state of the exhaust gas, the NOx occluding material 23, which occluded nitrogen dioxide (NO2) and changed into nitride, releases the nitrogen dioxide (NO2) that it has occluded and returns to the original barium (Ba) and the like, as shown in FIG. 5. As oxygen (O2) is absent in the exhaust gas, this released nitrogen dioxide (NO2) is reduced on the catalyst metal using carbon monoxide (CO), hydrocarbon (HC) and hydrogen (H2) in the exhaust gas as reducer. As a result, nitrogen dioxide (NO2) is transformed into nitrogen (N2), water (H2O), and carbon dioxide (CO2) and cleaned.
In this NOx occlusion reduction type catalyst, however, there is a problem that the NOx purifying efficiency falls as driving continues because sulfur (sulfur component) in the fuel is accumulated in the NOx occluding material in the catalyst. Consequently, as described in Japanese Patent Laid-Open 1998-274031, it is necessary to perform substantially periodical sulfur purges (sulfur component desorption) by setting the temperature of the exhaust gas flowing into the catalyst at approximately 600° C. to 650° C. or more and in rich atmosphere, though different depending on catalysts.
In this sulfur purge, there is a problem that carbon monoxide (CO) is discharged outside the engine.
In other words, in this sulfur purge, sulfur (S) is absorbed in the NOx occluding material in the form of nitride. Consequently, sulfur component (S) is released as sulfur dioxide (SO2), by transforming sulfate into carbonate with carbon monoxide (CO), in an oxygen-free and high temperature state. For this reason, the oxygen-free and high temperature state is realized by putting the exhaust gas in a rich air-fuel ratio state and by raising the temperature of the catalyst in case of diesel engines. This rich air-fuel ratio state is realized by reducing the exhaust quantity through intake throttling, large quantity of EGR and the like, and by performing post-injection, direct gas oil addition to the exhaust pipe, and the like. In addition, the temperature rise of the catalyst is realized by heating the catalyst with the heat generated by the oxidation of added fuel through the catalytic function.
In this hot rich atmosphere, carbon monoxide (CO) is produced by partial decomposition of hydrocarbon (HC), fuel; namely, through combustion of hydrogen component. On the other hand, in the NOx occluding material, nitrogen dioxide (NO2) is released more actively than sulfur dioxide (SO2) because nitride reacts more with carbon monoxide (CO) and changes into carbonate, compared to sulfate.
For this reason, in the prophase of the rich air-fuel ratio state performed by this sulfur purge control, reactions as follow will occur. Though sulfur dioxide (SO2) is released from the NOx occluding material 23, nitrogen dioxide (NO2) is released more actively. As a result, as shown in FIG. 5, carbon monoxide (CO) is used for reduction and removal of NOx to be released. In addition, carbon monoxide (CO) reacts with oxygen (O2) released by reduction of this nitrogen dioxide (NO2). Hence, carbon monoxide (CO) is not discharged outside the engine.
However, in the later stage of the rich air-fuel ratio state as the sulfur purge progresses, reactions as follow will occur. Though the release of sulfur dioxide (SO2) from the NOx occluding material 23 is sustained, the release of nitrogen dioxide (NO2) almost terminates in the latter stage of the rich air-fuel ratio state. As a result, as shown in FIG. 6, carbon monoxide (CO) is no longer used for reduction of nitrogen dioxide (NO2), and oxygen (O2) released by this reduction also decreases. Then, the oxygen concentration falls rapidly and the carbon monoxide concentration increases. As a result, carbon monoxide (CO) is discharged outside the engine.
FIG. 7 schematically shows the circumstances of this rapid decrease of oxygen concentration by the upstream excess air factor λ(u) and the downstream excess air factor λ(d) of the NOx occlusion reduction type catalyst. FIG. 8 shows examples of time series of upstream oxygen concentration (O2(u)) and downstream oxygen concentration (O2(d)) of the NOx occlusion reduction type catalyst, sulfur dioxide (SO2), and carbon monoxide (CO), in the sulfur purge of the prior art. It can be observed in the T1 portion indicated by an arrow that the upstream oxygen concentration (O2(u)) decreases rapidly and carbon monoxide (CO) increases.